The present invention generally relates to circuitry for supplying current to electrical devices. More specifically, the present invention relates to circuitry for supplying a constant current to airport signs so as to maintain a uniform brightness despite varying input currents.
Typically, airports or airfields have numerous electrical devices and systems which are used to guide airplanes and personnel through the increasingly complex maze of runways and taxiways. Such electrical devices include, for example, lighting systems which outline the runways and taxiways, as well as signs used to direct traffic or identify areas.
Airfield series circuits are usually provided with different brightness level amperage steps so that the brightness of the lights which outline the runways and taxiways can be adjusted to provide a dimming function. For example, taxiway edge light circuits may have a fixed intensity requiring 6.6 amperes or they may have three intensity steps at 6.6 amperes, 5.5 amperes, or 4.8 amperes. On the other hand, runway edge light circuits usually have five step settings of 6.6, 5.2, 4.1, 3.4, and 2.8 amperes. This allows the brightness of these various lights to be adjusted from the control tower as needed.
The difficulty with this arrangement is that the airfield signs are usually also driven by the same series circuitry that feeds current to the runway and taxiway lights. Since the FAA requires that the light output from airport signs remain at a constant level, even when the circuits to which they are attached change current levels, designers of airfield series circuits have been forced to adapt these circuits to provide this required functionality.
Conventionally constant brightness of airfield signs has been achieved by providing a transformer associated with each of the signs that saturates over a relatively large range of input current values and an associated constant current regulator. A problem with this solution is that it provides a resultant power factor of less than 0.5 which greatly reduces the cost efficiency of the system.